1. Field of the Invention
This invention relates to a method of manufacturing a one-chip microcomputer with ROM mounted therein.
2. Description of Related Art
In general, there are two types of ROMs mounted in a one-chip microcomputer, namely, a mask ROM for which informations to be stored are decided in the manufacturing process by using photomask, and an EPROM or E.sup.2 PROM allowing electrical writing of informations after the manufacture. Although the mask ROM is advantageous from the viewpoint of manufacturing cost in a mass production, it takes a long time to complete a sample after the informations to be stored are decided, that is, turnaround time is lengthy. Moreover, the photomask costs too much in a small quantity of production. Meanwhile, informations can be written into the EPROM or E.sup.2 PROM after the manufacturing process, so that the turnaround time is remarkably shortened, contributing to a reduction of manufacturing cost in a small quantity of production. On the other hand, however, the manufacturing process of a wafer becomes complicated thereby to raise the manufacturing cost of the EPROM or E.sup.2 PROM as a whole, and therefore the EPROM or E.sup.2 PROM is disadvantageous if in a mass production.
As noted above, the mask ROM and EPROM or E.sup.2 PROM have their own merits and demerits. Therefore, a plurality of kinds of chips with mask ROM, EPROM E.sup.2 PROM, etc. have been developed in recent years for microcomputers.
For a ROM mounted in a one-chip microcomputer with mask ROM, for example, a depletion type ROM, contact type ROM, OR type ROM through ion implantation or the like is used. The line decoder, string decoder and sense amplifier which have circuits and characteristics corresponding to the kind of the using ROM are used. On the other hand, for a ROM mounted in a one-chip microcomputer with EPROM or E.sup.2 PROM, an OR type ROM is generally used, and the line decoder, string decoder and sense amplifier are of the type corresponding to this OR type ROM. Besides, the one-chip microcomputer with EPROM or E.sup.2 PROM requires particularly a writing circuit and a controlling circuit, etc.
It is natural that the one-chip microcomputer with mask ROM and that with EPROM or E.sup.2 PROM are different from each other in layout and size, whereby the photomask should be fabricated individually for each chip, bringing about such problems as an elongated time required for development and cost therefor. Furthermore, although it is necessary that the former microcomputer with mask ROM and the latter microcomputer with EPROM or E.sup.2 PROM have the same electric characteristics from the sense of compatibility, it is difficult to make the electric characteristics equal to each other when the layout and size of the chips are not the same.
FIG. 1 shows the layout of a conventional one-chip microcomputer with EPROM, and FIG. 2 shows the layout of a conventional one-chip microcomputer with mask ROM. Referring to FIGS. 1 and 2, numerals 1 and 11 represent a memory array region, 2 and 12 representing a line decoder region, and 3 and 13 representing a region including a string decoder and a sense amplifier, respectively. It is clear from these FIGS. 1 and 2 that the layout and size of the one-chip microcomputer with mask ROM are different from those of the one-chip microcomputer with EPROM. This goes true also between the one-chip microcomputer with mask ROM and a one-chip microcomputer with E.sup.2 PROM.